Ultraviolet (UV) radiation is ubiquitous, the sun being the most common source of UV radiation although not the only source. As UV radiation can cause damage to people, animals and objects, compositions that provide protection from UV radiation are useful.
In the biological context, UV-protective compositions, i.e. compositions that reduce or block the transmission of UV rays, are commonly employed to protect against sunburn. Sunburn is a form of radiation burn resulting from an overexposure to UV radiation, typically from the sun, but also from artificial sources, such as tanning lamps, welding arcs, and ultraviolet germicidal irradiation. Normal symptoms of sunburn in humans and other animals include reddening of the skin, general fatigue and mild dizziness. An excess of UV radiation can be life-threatening in extreme cases. Excessive UV radiation is considered to be the leading cause of non-malignant skin tumors, as well as increasing the risk of certain types of skin cancer.
Sunscreen compositions are commonly used to prevent sunburn and are believed to prevent squamous cell carcinomas and melanomas. Furthermore, they have been reported to delay the development of wrinkles and additional age-related skin conditions.
Specifically, sunscreen compositions are topical compositions that include components that absorb and/or reflect at least some of the sun's UV radiation on areas of skin exposed to sunlight, and thus reduce the effect of UV radiation on the skin. Depending on their mode of action, they are typically classified as chemical or physical sunscreens.
Chemical sunscreen compositions comprise organic compounds that absorb UV radiation to reduce the amount of UV radiation that reaches the skin. Being transparent to visible light and thereby being invisible when applied to the skin, chemical sunscreen compositions are popular for use. However, some organic compounds used in chemical sunscreen compositions have been found to generate free radicals which can cause skin damage, irritation and accelerated aging of the skin. Furthermore, organic materials may be absorbed into the skin, resulting in long-term detrimental health effects. Chemical sunscreen compositions may require the addition of a photostabilizer.
Physical sunscreen compositions reflect and absorb UV radiation. Known physical sunscreen compositions comprise particles of inorganic materials, mainly titanium oxide and/or zinc oxide. In order to obtain absorption and/or reflection of ultraviolet radiation over the full UVA and UVB range, relatively large particles are used. Due to the large particle size, such sunscreen compositions are viscous and opaque and tend to leave a white cast on the skin.
Many sunscreen compositions protect against UV radiation in the 280-315 nm range (UVB radiation) that causes sunburn, but do not against UV radiation in the 315-400 nm range (UVA radiation), which does not primarily cause sunburn but can increase the rate of melanoma and photodermatitis.
It is generally preferred that sunscreen compositions, when applied to the skin, appear transparent to the eye. In order for physical sunscreen compositions to appear transparent to the eye, the particles of inorganic material should be in the form of nanoparticles, which absorb and/or scatter UV light but not visible light, rendering them substantially transparent to the eye when applied on the skin. However, use of nanoparticles reduces the range of wavelengths absorbed by the inorganic materials. Some known sunscreen compositions therefore block both UVA and UVB radiation by use of a combination of different UV-absorbing or scattering materials, generally termed UV-protecting agents, each of which blocks radiation over a limited range of the UV spectrum.
Similarly, UV-protective compositions can benefit inert materials or objects that may be negatively affected by UV radiation. For instance, UV radiation can reduce the life-span of materials (e.g., natural and synthetic polymers), and exposure to UV radiation may cause changes in colors of objects, especially in objects that are subjected to prolonged sun exposure, such as buildings or vehicles. Various coatings are known to achieve such protection. The provision of such coatings may in turn benefit health. For example, optical lenses having a UV-protective coating may reduce the transmission of such radiation to the eye, thus reducing UV-induced optical disorders such as cataract. Similarly, materials serving for the fabrication of windows that incorporate or are coated with suitable UV-protecting agents may reduce the transmission of such rays to subjects, plants or objects shielded by such windows.